1. Field of the Invention
The present invention relates to a method and apparatus of continuously casting a metal sheet by using cooling members such as cooling drums and belts that are movable and act as a part of a mold, and more particularly, to a continuous casting method and apparatus that produces a metal sheet having a high quality and superior surface characteristics.
2. Description of the Related Art
A reduction in manufacturing costs and a creation of new materials are particularly required in the field of continuous metal casting, and accordingly, there is a strong demand for the ability to cast a metal sheet one to ten millimeters in thickness that is nearly equal to the thickness of a final product, by using, for example, a drum-type continuous casting machine incorporating a cooling mechanism. This sort of technique is disclosed in Japanese Unexamined Utility Model Publication 58-157250, Japanese Unexamined Patent Publication 60-184449, Japanese Unexamined Patent Publication 1-83340, Japanese Unexamined Patent Publication 1-83342 and Japanese Unexamined Patent Publication 62-130749, etc.
The object of Japanese Unexamined Patent Publication 60-184449 Japanese Unexamined Patent Publication 1-83340, Japanese Unexamined Patent Publication 1-83342 is to equalize the solidified thickness of a cast metal sheet and prevent surface cracks, by providing irregularities of cooling on the surface of a cooling drum. Japanese Unexamined Patent Publication 62-130749 prevents an inclusion of oxides in a cast metal sheet, and a deterioration of the surface quality of the cast metal sheet, by casting molten metal in an inert gas atmosphere.
According to tests carried out by the inventors of the present invention, however, these conventional techniques do not substantially provide sheets having a good and stable surface quality. For example, Japanese Unexamined Patent Publication 60-184449 forms irregularities, i.e., recesses and protrusions each about four micrometers or more in size on the surface of a drum, but this disclosure does not pay careful attention to the relationship between the surface irregularities and the thickness of a cast metal sheet, and thus the problems of surface cracks and a surface quality deterioration may arise. Namely, when the cast metal sheet is thin, and the irregularities formed on the surface of the cooling drum are too large compared with the thickness of the sheet, thermal stress may be concentrated around the irregularities to thereby produce small cracks that remain as surface defects of the sheet. On the other hand, when the cast metal sheet is thick and the irregularities are too small compared with the thickness of the sheet, the solidification stress is not sufficiently distributed and therefore, large surface cracks are produced.
Japanese Unexamined Patent Publication 62-130749 is also not satisfactory because the rotary mold thereof does not have surface irregularities, and therefore, cooling may increase an amount of thermal contraction to cause a local stress concentration, to thereby produce surface cracks.